1. Field of the Invention
This invention relates generally to a system that detects x-rays using active pixel sensor arrays.
2. Discussion of the Related Art
X-ray detection has long served as a useful diagnostic tool in a wide variety of fields. In the medical field, for example, x-ray detection has been used to capture images representing parts of a patient's body, which images are then used in diagnosis and treatment. X-rays have also long been used in bone densitometry (i.e., the measuring of bone density). In the dental and related fields, x-rays are used to take images of a patient's teeth or other areas of a patient's mouth. X-ray detection is also used in industrial fields, such as, for example, to detect defects in pipe welds or aircraft joints, or to perform non-destructive testing on materials such as ceramics or glass bottles. X-ray detection is also used in spectroscopy, to determine such things as the crystal spacing or particle composition of a material under test. X-ray detection is also used in the surveillance and security fields, such as, for example, in an airport luggage scanning system. X-ray detection is used in other fields as well.
The most conventional x-ray detection techniques use photosensitive film to register an image. For example, in conventional dental x-ray detection, a film cartridge is placed in the patient's mouth. The film is exposed to x-rays which have passed through the soft tissue of the patient's mouth. Chemical development of the film in the cartridge produces an image which provides information that assists the dentist in making a diagnosis and providing appropriate treatment.
Film is used in medical x-ray detection as well. For example, a chest x-ray can be taken by placing a large piece of film in front of the patient, and passing x-rays through the patient's body from the back. Here again, the film must be developed using chemicals to provide an image that is useful in making a diagnosis and providing treatment to the patient.
The drawbacks of using film to register the x-ray image are well known. Foremost among these are the cost and inconvenience involved in developing the film to obtain an image. To begin with, the development process requires the use of chemicals. Such chemicals are expensive, troublesome to store and can also have a negative impact on the environment. The development process is also time consuming. The x-ray technician, operator or physician, after exposing the film to x-rays, must develop it in a darkroom or a closed processor.
In dental radiography, the relatively high dosage of radiation which the patient must receive to expose the film sufficiently is also a major drawback. Although the average radiation dosage per exposure using film has been reduced over the years, the maturity of the conventional film technology would suggest that further significant decrease in the required dosage is unlikely.
In view of the above problems, a number of methods of x-ray imaging have been proposed which do not require the use of film. Many of these systems operate by converting the x-rays, by use of a scintillator, into visible light, and subsequently converting the visible light into electrical signals which can be processed by electronic circuits to create an image on a display to form an image.
In the intraoral radiography field, for example, the pioneer patent is U.S. Pat. No. 4,160,997, issued to Dr. Robert Schwartz and hereby incorporated by reference. Other examples include U.S. Pat. No. 5,434,418, issued to David B. Schick and assigned to the assignee of the present application, and U.S. Pat. No. 4,987,307, issued to Giorgio Rizzo and Cesare Gadda.
Each of these patents describes a x-ray detector which includes a scintillator screen, and a separate and distinct Charge-Coupled Device (CCD). The scintillator screen converts the x-rays emerging from a radiated tooth into visible light, while the CCD converts the light into electrical signals. These devices, while solving many of the problems with photographic film, have problems inherent to their design.
These problems stem from the use of a charge-coupled device (CCD) as the image conversion device. In a CCD, packets of electrical charges are stored in one of an array of discrete locations (known as "pixels"), with the amount of charge created and stored in each pixel corresponding to the intensity of light hitting the device at that location. The amount of charge stored in each pixel is read out by the successive application of control voltages to the device, which control voltages cause the packets of charge to be moved from pixel to pixel to a single output circuit. Through this process, the output circuit produces an analog electrical signal the amplitude of which at a given point in time represents the intensity of light incident on the device at a particular correspondence spatial location.
A CCD relies in its operation on the transfer of electrons from one pixel to another, a process that is often analogized to a "bucket brigade." Accordingly, before reaching the output circuit, the transferred electrons must pass though silicon for macroscopic distances, on the order of centimeters. Because of this, the ratio of electrons successfully transferred to the number left behind per electrode, the so-called "charge transfer efficiency" (CTE), must be as close as possible to perfect (i.e., no electrons left behind) to ensure acceptable performance of the CCD.
In addition, since net CTE varies exponentially with the number of charge transfers, the requirement for transfer efficiency becomes more stringent as CCD array sizes become larger. Also, manufacturing yield may decrease as the array size increases, since CCDs are vulnerable to single point defects that can block an entire column, rendering the entire device unusable. CCDs also require special manufacturing techniques to achieve the required high CTE. As a result of the necessity of using such techniques, CCDs are not integratable with low power CMOS circuits, the technology most appropriate for low power integration of on-chip timing and driver electronics that is required for instrument miniaturization. Moreover, since CCDs require 12-26 volts of power, devices using this technology can present something of a shock hazard.
Other devices have also been used as the image conversion device in lieu of CCDs. For example, U.S. Pat. No. 5,043,582 to Cox et al. describes an x-ray imaging system constructed from a light sensitive dynamic random access memory (DRAM). The device is non-monolithic, consisting of a first layer of light sensing elements (a "focal plane array") and a second layer of transistors for reading out data from the light sensing elements, with the layers interconnected with indium bump bonds. The non-monolithic nature of these structures, however, inherently causes a number of problems. To begin with, the fabrication processes for such devices are very complex and low yielding, making the systems expensive to produce. Further, the separate layers thermally expand and contract at different rates, resulting in reliability problems with the device. In addition, the passive nature of these devices (i.e., the absence of an active transistor within the pixel unit cell) results in a high readout noise.
X-ray sensor arrays have also been made of amorphous silicon. Such devices comprise generally an array of amorphous photodiodes, and an array of thin-film transistors which select the photodiodes that are to be read out. Such devices, however, are passive, and, like the system described in the Cox et al. patent, suffer from high readout noise. In addition, there are limitations as to how small pixels in non-monolithic devices can be made, since advanced photo-lithographic techniques cannot be used. These pixel size limitations in turn limit the resolution that can be achieved.
Recently, Active Pixel Sensor (APS) technology has provided an alternative to CCDs and other sensing devices for converting light into electrical signals. This technology is shown, for example, in U.S. Pat. No. 5,471,515 to Fossum et al., and hereby incorporated by reference. In general terms, an APS array is defined as an array of light sensors having one or more active transistors associated with each pixel. The transistors, which are the pixel's "active" elements, perform gain or buffering functions.
Because each pixel has its own active element, the charges that collect below each photosite need not be transferred through a "bucket brigade" during the readout period, as in a CCD. Thus, the need for nearly perfect charge transfer is eliminated. Accordingly, an APS array does not exhibit the negative attributes associated with charge transfer across macroscopic distances required by the CCD.
Also, since APS devices can be manufactured using standard CMOS techniques, the array can operate on 5 volt power, minimizing the shock hazards of the device. An additional advantage of utilizing APS technology in x-ray applications is that CMOS wafers are made in much larger diameter than are CCD wafers. This would allow for the manufacture of very large devices for other radiology applications, such as mammography, fluoroscopy, orthopedics, etc.
While APS arrays have of late enjoyed a good deal of attention from those constructing light detecting devices--such as in the high definition television (HDTV) and electronic still camera fields--they have not heretofore been used to construct an x-ray detector. The reasons for this are several. To begin with, an x-ray detector is generally constructed by disposing a scintillator on top of a light sensing device, so that the scintillator first converts incident x-rays into visible light, and the light sensing device in turn converts the visible light into electrical signals. Some fraction of the x-rays that enter the scintillator, however, will invariably exit the scintillator and impinge upon the light sensing device. Such unconverted x-rays would be registered by conventional APS devices, and cause spurious signals to be created, which would, in turn, result in a noisy image.
In addition, the visible light emitted by scintillators is typically in the blue-green portion of the visible spectrum. APS arrays, however, are widely believed to exhibit a very poor response to blue-green light, leading in turn to the belief that APS arrays are not suitable for use in x-ray detectors.
Another problem with x-ray detectors is event detection. Since x-ray detectors are generally manufactured separately from, and not synchronized with, the source of x-rays, the x-ray detector must have some mechanism for determining when it has been exposed to x-ray rays, so that it knows when to read out the data. This problem does not exist with the devices such as digital cameras, since the visible radiation which cameras sense is either always present or is provided from a flash that is synchronized with the operation of the camera.
Also, APS devices have a higher dark signal (i.e., thermally generated currents produced by the device when not exposed to radiation) than CCDS, since the dark signal in CCDs can be significantly reduced by operating the device in the multi-phase pinned (MPP) mode. This is believed to make APS arrays less suitable as x-ray detectors than as light detectors. In particular, because scintillators emit a much smaller number of photons than are present in a light sensing environment (such as, for example, a photography environment), the dark signal is believed to be more problematic in an x-ray detector, since the dark signal, if not corrected for, will have a greater impact on the signal-to-noise ratio.
Furthermore, it has been theorized that CMOS transistors, which are the type used in constructing APS devices, are more susceptible to damage and noise generation from high frequency radiation such as x-rays than the MOS transistors used in CCDs. Still further, it has been theorized that large APS arrays will have poor manufacturing yields.
There is a need, therefore, for a new type of x-ray detector that solves the problems of conventional x-ray detectors by exploiting APS technology, while at that same time overcoming the real and perceived drawbacks associated with using APS arrays to detect x-rays.